The present invention relates to low stress relaxation elastomeric materials suitable for use in macroscopically-expanded, three-dimensional, apertured polymeric webs.
It has long been known in the field of disposable absorbent articles that it is desirable to construct absorptive devices, such as disposable diapers with fasteners, pull-on diapers, training pants, sanitary napkins, pantiliners, incontinent briefs, and the like, with elastic elements to improve the range of size, ease of motion, and sustained fit. It is also well known that it is preferable, especially in such products intended to be worn in hot and humid conditions, to provide adequate porosity to all areas of the article where undue occlusion of the skin may cause sensitized skin or heat rash. Due to the nature of many disposable absorbent articles there is a high potential for skin irritation due to trapping of moisture and other body exudates between the elasticized portion of the article and the skin of the wearer. Elasticized portions of disposable articles are particularly prone to causing skin irritations as they tend to be more conformable to the body, and therefore more likely to occlude areas of the skin, often for long periods of time. Various methods are known in the art for imparting elasticity to polymer films. As materials with greater elasticity provide health care or personal hygiene products with a better fit to the body, the air flow to the skin and the vapor flow from the occluded areas are reduced. Breathability (particularly vapor permeability) becomes more important for skin health. Various methods are also known in the art for imparting porosity to polymer films to improve breathability, but there remains a need for a polymeric film or web that provides for both adequate elasticity and porosity, such as may be adapted for durable, prolonged use in personal hygiene or health care products, particularly disposable articles, bandages, wraps, and wound dressings.
Disposable diapers and other absorbent articles fitted with elasticized leg cuffs or elasticized waist bands for a more comfortable fit, as well as providing for better leakage control, are known in the art. Often, the elasticity is accomplished with a heat treatment of polymeric materials that results in a desirable shirring or gathering of a portion of the diaper. One such method of treatment is disclosed in U.S. Pat. No. 4,681,580, issued to Reising et al. on Jul. 21, 1987, and hereby incorporated by reference herein. Other methods for imparting elasticity are taught in U.S. Pat. No. 5,143,679 issued to Weber et al. on Sep. 1, 1992, U.S. Pat. Nos. 5,156,793 issued to Buell et al. on Oct. 20, 1992 and 5,167,897 issued to Weber et al. on Dec. 1, 1992, all are hereby incorporated by reference herein.
Several means of rendering elasticized planar polymer films more porous are known in the art, such as die punching, slitting, and hot-pin melt aperturing. However, when any of the above techniques is applied to thermoplastic elastomeric films, the increase in porosity is accompanied by a decrease in the degree of reliable elastic performance. For example, in the case of circular apertures in a planar film, it is well known that for an applied stress S1, a resultant local stress, S2, is created orthogonal to the applied stress about the apertures. This local stress, S2, is greater than S1, approaching a magnitude up to 3 times the applied stress. For non-round apertures the concentration of stress can be even greater. As a result, apertures become sources of tear initiation sites at their edges, because the edges of the material form the edges of the apertures in the plane of applied stress. For common thermoplastic elastic films, such apertures facilitate tear initiation which can propagate over time leading to catastrophic failure of the film. When used in elasticized portions of disposable absorbent articles, this failure results in the loss of important elastic characteristics, including loss of comfort, fit and use of the absorbent article.
Prior art web structures that do provide adequate porosity so as to be preferable for use as the wearer-contacting surface on disposable absorbent articles have been of two basic varieties, i.e., inherently fluid-pervious structures, such as fibrous nonwovens, and fluid-impervious materials such as polymeric webs which have been provided with a degree of fluid permeability via aperturing to permit fluid and moisture flow therethrough. Neither variety is characteristically elastic, and as a result both are generally used in regions of an absorbent article requiring fluid permeability but not extensibility, such as the body-contacting layer of a catamenial pad.
Commonly assigned U.S. Pat. No. 3,929,135 issued to Thompson on Dec. 30, 1975, and hereby incorporated herein by reference, suggests a suitable body-contacting porous polymeric web for disposable articles. Thompson teaches a macroscopically-expanded, three-dimensional topsheet comprised of liquid-impermeable polymeric material. However, the polymeric material is formed to comprise tapered capillaries, the capillaries having a base opening in the plane of the topsheet, and an apex opening in intimate contact with the absorbent pad utilized in the disposable absorbent article. The polymer material taught by Thompson is not generally an elastomer, however, and Thompson depends on the inelastic properties of the heat-molded single layer film to produce the desired three-dimensional structure.
Still another material which has been utilized as a body contacting surface in a disposable absorbent article context is disclosed in commonly assigned U.S. Pat. No. 4,342,314 issued to Radel et al. on Aug. 3, 1982, and hereby incorporated herein by reference. The Radel et al. patent discloses an improved macroscopically-expanded three-dimensional plastic web comprising a regulated continuum of capillary networks originating in and extending from one surface of the web and terminating in the form of apertures in the opposite surface thereof. In a preferred embodiment, the capillary networks are of decreasing size in the direction of liquid transport.
The macroscopically-expanded three-dimensional plastic webs of the type generally described in the aforementioned commonly assigned Thompson and Radel et al. patents have met with good success in permitting adequate vapor permeability due to the porosity provided by the apertures. However, because of material limitations such webs do not generally possess the requisite elasticity to allow the resulting web to have significant elastomeric characteristics. This shortcoming substantially limits the use of such webs in elasticized portions of an absorbent article.
Elasticized polymeric webs may be produced from elastomeric materials known in the art, and may be laminates of polymeric materials such as disclosed in U.S. Pat. No. 5,501,679, issued to Krueger et al. on Mar. 26, 1996. Laminates of this type are generally prepared by coextrusion of elastomeric materials and inelastic skin layers followed by stretching the laminate past the elastic limit of the skin layers and then allowing the laminate to recover. Elastomeric webs or films such as those described above may be used in the body hugging portions of garments, such as the waistbands, leg cuffs and side panels, but are generally not porous enough to prevent undesirable skin irritations when used for prolonged periods of time.
Additionally, the actual use condition for absorbent articles or other personal care products typically involves heat, humidity, loading or combinations thereof. Some elastomeric materials suffer loss of elastic properties and dimensional stability at body temperature, especially under load or tension. The loss of elastic properties and dimensional stability results in sagging and ill-fitting of the absorbent article, and in severe cases, leakage from the absorbent article may result.
The elastic components of certain articles, such as training pants, pull-on diapers, disposable diapers with fasteners, adult incontinence garments, bandages, wraps, wound dressings, and the like, may be subjected to considerable amount of stretching, at a strain as large as 400% of its original dimension, while the article is being put on the body of the wearer. This step imposes additional requirements of stretchability and recoverability on the elastomeric material
There is considerable difficulty in processing and handling elastomeric materials, due to the inherent tacky and stretchy nature of the elastomeric materials. The elastomeric materials have a tendency to stick to the processing equipment, and are difficult to remove from a roll or cut to the correct size to be incorporated into the finished products.
Therefore, it is desirable to provide an elastomeric material which substantially retain its elastic properties under actual use condition of the finished product over a specified period of time, for example, at body temperature under sustained load for about 10 hours.
It is desirable to provide such an elastomeric film that is form-fitting and breathable (i.e., vapor permeable).
More particularly, in a particularly preferred embodiment, it would be desirable to provide a macroscopically-expanded three-dimensional apertured elastomeric web that is able to substantially recover its three-dimensional shape after being subjected to an applied strain of up to about 400% or more.
It is further desirable to provide an elastomeric film suitable for use in an apertured elastomeric web designed to dissociate the effects of an applied strain on the web from the edges of the apertures and hence retard or prevent the onset of tear initiation.
Furthermore, it is desirable to provide such an elastomeric material that has improved processability and is cost-effective for personal hygiene products and health care products, such as pull-on diapers, training pants, disposable diapers with fasteners, incontinence garments, sanitary napkins, pantiliners, wound dressings, bandages, and wraps.
The present invention pertains to a low stress relaxation elastomeric material. The elastomeric material can be used alone or with skin layers to form an elastomeric film. The elastomeric film is useful in a forming process to provide a porous, macroscopically-expanded, three-dimensional, elastomeric web. In a preferred embodiment, the elastomeric web is suitable for use in elasticized or body-hugging portions of disposable absorbent articles such as side panels, waist bands, cuffs, or of health care products such as dressings, bandages and wraps. The porous extensible polymeric webs of the present invention may also be used in other portions of the absorbent articles where a stretchable or breathable material is desired, such as topsheets or backsheets.
The elastomeric materials of the present invention preferably exhibit low stress relaxation at body temperature and under load or stress for a specified period of time. The elastomeric materials also exhibit low hysteresis and high elongation at break when subjected to a large deformation. In a preferred embodiment, the elastomeric material comprises a styrenic block copolymer such as polystyrene-poly(ethylene/propylene)-polystyrene (S-EP-S), polystyrene-poly(ethylene/butylene)-polystyrene (S-EB-S), polystyrene-polybutylene-polystyrene (S-B-S), polystyrene-polyisoprene-polystyrene (S-I-S), or hydrogenated polystyrene-poly(isoprene/butadiene)-polystyrene (S-IB-S) at least one vinylarene resin, and a processing oil, particularly a low viscosity hydrocarbon oil such as mineral oil.
The elastomeric materials of the present invention may be in a monolithic film or in a multilayer film with at least one substantially less elastomeric skin layer such as polyolefin type materials, including polyethylene and polypropylene. The elastomeric films are useful in forming macroscopically-expanded, three dimensional, elastomeric webs.
In a preferred embodiment, the web has a continuous first surface and a discontinuous second surface remote from the first surface. The elastomeric web exhibits a multiplicity of primary apertures in the first surface of the web, the primary apertures being defined in the plane of the first surface by a continuous network of interconnecting members, each interconnecting member exhibiting an upwardly concave-shaped cross-section along its length. In a preferred embodiment each interconnecting member exhibits a generally U-shaped cross-section along a portion of its length, the cross-section comprising a base portion generally in the plane of the first surface of the web and sidewall portions joined to each edge of the base portion and interconnected with other sidewall portions. The interconnected sidewall portions extend generally in the direction of the second surface of the web, and are interconnected to one another intermediate the first and the second surfaces of the web. The interconnected sidewall portions terminate substantially concurrently with one another to form a secondary aperture in the plane of the second surface of the web.
When used as an extensible, porous member in an absorbent article, the elastomeric layer of the present invention allows the interconnecting members to stretch in the plane of the first surface. The three-dimensional nature of the web allows the strain on the interconnecting members in the plane of the first surface to be dissociated from the strain at the secondary apertures in the secondary surface, and therefore decoupled from potential strain-induced stress at tear initiation sites. This dissociation, or decoupling, of the strain-induced stress of the web from strain-induced stress at the secondary apertures significantly increases web reliability by allowing repeated and sustained web strains of up to about 400% or more without failure of the web due to tear initiation at the apertures.
Also disclosed is a method of producing the elastomeric web of the present invention which comprises providing a multilayer elastomeric film, supporting the film on a forming structure, and applying a fluid pressure differential across the thickness of the multilayer film. The fluid pressure differential is sufficiently great to cause the multilayer film to conform to the supporting structure and rupture in at least portions of the formed film.